Peter Frost's anthropology blog, with special reference to sexual selection and the evolution of skin, hair, and eye pigmentation

Saturday, November 6, 2010

Extraversion: a tool for mating success

Extraversion is part of the male toolkit for mating success. It is especially useful in societies where a high incidence of polygyny means too many men must compete for too few women.

As a single man, I would spend close to $3,000 a year on dating. And that didn’t include things like buying a sportier-looking car. My behavior was also higher risk, in large part to impress women and show how ‘edgy’ I could be.

The mating game is costly. A British study found that men improve their sexual access to women at the cost of increased risk of hospitalization for accident or illness (Nettle 2005). There is thus a trade-off. If you invest more effort in chasing women, less is left over for taking care of yourself and any children you’ve fathered. You also risk early death.

The trade-off varies from one society to another. For a man, the mating game is less costly if the woman can provide for herself and her children with little male assistance. The cost may even be negative if she also provides for her man.

In such societies, it is in a man’s reproductive interest to mate with as many women as possible. The result? A ‘tragedy of the commons’ where the pursuit of self-interest ends up harming society as a whole. There won’t be enough women to go around, and many men will go a long time without a mate, or never find one at all.

This has long been the case with simple ‘horticultural’ societies in the tropical zone. The women feed themselves and their children with minimal male assistance because they can grow food year-round. And this food production is not appropriated by a State or a land-owning class.

Such societies remain simple in large part because intense sexual competition keeps them from evolving into more complex entities. The surplus males stir up endless conflict, if only because their sole access to women is through warfare, i.e., rape and abduction. There can never be pacification and, therefore, the formation of larger, more advanced societies.

How men and women adapt to high-polygyny environments

A high incidence of polygyny favors men with a different toolkit of physical and mental traits. Some personality traits, for instance, will be more advantageous than others. Such is the finding of a series of studies from rural Senegal, where 48% of men over 40 are polygynous.

Men

Alvergne et al. (2009, 2010a, 2010b) found no correlation among Senegalese men between mating success and most personality traits, i.e., neuroticism, openness, and agreeableness. One trait, however, showed a strong correlation. This was extraversion, defined as “pro-social behavior which reflects sociability, assertiveness, activity, dominance and positive emotions.” Men with above-medium extraversion were 40% more likely to have more than one wife than those with below-medium extraversion, after controlling for age. Furthermore, this personality trait correlated with higher testosterone levels. Such a linkage suggests that extraversion is part of the male toolkit for mating success in a high-polygyny environment.

Of course, it may be that the relation of cause and effect is indirect. Extraversion helps men accumulate wealth, and wealthy men can afford to take second or third wives. Nonetheless, the correlation remained significant even after the researchers controlled for social class

Women

Among Senegalese women, reproductive success correlated with neuroticism, i.e., a tendency “to be anxious, depressive, and moody.” Women with above-medium neuroticism had 12% more children than those with below-medium neuroticism, after controlling for age and marital rank. This personality trait may thus be part of the female toolkit for infant survival in an environment where women are almost solely responsible for parenting.

Could the cause and effect run in the other direction? Perhaps having more children makes a woman more neurotic. Yet neuroticism did not increase with age, whereas the number of children did. Furthermore, the correlation was stronger among rich women, who presumably had less reason to worry about child care.Inter-individual variation

On average, the Senegalese men were more oriented to polygyny and extraversion, but there was significant variation. Some seemed to be more monogamous and introverted.

Perhaps all human populations display this sort of variation in reproductive strategy, the differences among them being one of degree than of kind. Indeed, statistical differences can easily develop among human populations because the raw material for gene-culture co-evolution is already available. There is no need to wait for new mutations to come into existence.

Inter-population variation

What do the authors say about differences among human populations? They initially state, “Men in the present study had lower T [testosterone] levels than has been recorded for men from western societies using similar saliva assays” (Alvergne et al. 2009). Later on, in reviewing the literature, they qualify this statement:

It is worthy of note that inter-population differences in T levels were found to be more pronounced for young men (15-30 years) than for older men (45-60 years) (Ellison, 2003). The authors conclude that the differences between populations in patterns of T decline with age result from variations in peak levels during young adulthood and are thus highly dependent on the reproductive physiology of young males.(Alvergne et al. 2009)

The decline in T levels with age is sharper among polygynous Senegalese men than among monogamous Senegalese men. In short, higher T levels in young adulthood mean lower ones later in life:

When men get older than 50, a reversed pattern is observed, with polygynously married men having lower T levels than monogamously married men.(Alvergne et al. 2009).

This may account for the authors’ initial statement that T levels were lower in Senegalese men than in Western men. The Senegalese subjects were 38.3 years old on average.

A similar reversal with age has been noted in U.S. studies. African Americans have a clear testosterone advantage over Euro-Americans only from puberty to about 24 years of age. This advantage then shrinks and eventually disappears at some point during the 30s (1). The pattern then seems to reverse at older ages (Ellis & Nyborg 1992; Gapstur et al. 2002; Nyborg 1994, pp. 111-113; Ross et al. 1986; Ross et al. 1992; Tsai et al. 2006; Winters et al. 2001).

No one really knows why. We know that too much testosterone early in life causes long-term harm, e.g., increased risk of prostate cancer. Perhaps there is also degradation of the body’s capacity to produce testosterone.

A hen is an egg’s way to make another egg. I remember being told that as a child. Perhaps we should now say: A man is a sperm’s way to make more sperm.

H/T to Tod.

Note

1. Tsai et al. (2006) found that baseline levels of both total and bioavailable testosterone were significantly higher in African Americans than in Euro-Americans with a median age of 33-34. Ellis and Nyborg (1992) found that African Americans had a slight but still significant (p=0.028) testosterone advantage over Euro-Americans among subjects with a median age of 38. It is difficult to identify the ‘tipping point’ because both studies used pools of subjects with wide age ranges.

9 comments:

Anonymous
said...

In such societies, it is in a man’s reproductive interest to mate with as many women as possible. The result? A ‘tragedy of the commons’ where the pursuit of self-interest ends up harming society as a whole.

Uh oh, you called women a commons. The feminists will not like that. Take cover.

There was an interesting show on the cable science channel about an experiment in Siberia. There, foxes are selected against agressive and fear behavior by selective breeding. It took the experiment only 3 generations to make a fox like a pet dog with no fear of human beings.The 'tamed' foxes also display neotenic (childish) characters. Bigger eyes (or smaller jaw), curly tail, puffy fur, smaller limbs, very similar to pet dogs and the researchers explain that a lack of testosterone slows down the fox development.I think that's pretty much a good proof for what Peter says here.

Women with above-medium neuroticism had 12% more children than those with below-medium neuroticism, after controlling for age and marital rank.

Maybe because they worry and take better care of their children, but that would be expected to result in a larger number of surviving children

Neuroticism being correlated with having more children would rather suggest that the most fertile women were more neurotic as a side effect of being more fertile, possible due to a 'more female' hormonal balance.

Or maybe the neuroticism helped women get more attention from the husband and made them somewhat more likely to get pregnant.

It's not women who are the 'commons' but rather mating opportunities. When men pay a high price for mating, we can maintain the illusion that mating opportunities are limitless. When the price is low (because someone else is paying), the result is scarcity. And unlike most markets, this kind of scarcity is not self-correcting.

Anon,

Thanks!

Ben10,

Yes, natural selection can move very fast if the genetic variation already exists. There's no need to wait for new mutations.

Tod,

I think infant survival is the key factor. Neurotic mothers are more likely to monitor their kids and keep them out of harm's way.

> Women with above-medium neuroticism had 12% more children than those with below-medium neuroticism, after controlling for age and marital rank.

Why correct for marital rank. Maybe neurotics had high marital rank, but maybe they had low. In any case, it's unrealistic to correct for marital rank, assuming you're trying to learn about nature - nature does not correct for this.

One shouldn't adjust for age either, unless one also factors in the odds of premature death or debility or infertility. Again, nature does not permit every woman to go from age X to Y intact.

Welcome to my blog! For the most part, this page will be an extension of my website, with comments relating to my research. But it will also branch out into more general discussions of human evolution.